Non-volatile memory devices retain their stored data even when their power supplies are turned off. Therefore, non-volatile memory devices have been widely used in computers, mobile telecommunication systems, memory cards and so on. Flash memory devices are widely used as non-volatile memory devices. Many flash memory devices employ memory cells having a stacked gate structure. The stacked gate structure includes a tunnel oxide layer, a floating gate, an inter-gate dielectric layer and a control gate electrode, which are sequentially stacked on a channel region. Film quality of the tunnel oxide layer should be improved in order to improve reliability and program efficiency of the flash memory cells, and a coupling ratio of the flash memory cell should be increased.
Recently, novel non-volatile memory devices, for example, resistance random access memory (RAM) devices have been proposed instead of flash memory devices. A unit cell of a resistance RAM device includes a data storage element having two electrodes and a variable resistive material layer interposed between the two electrodes. The variable resistive material layer, that is, a data storage material layer, may be changed into a conductor or an insulator according to a polarity and/or a magnitude of an electrical signal (voltage or current) applied to the electrodes. In other words, the data storage material layer has a switchable state. The voltage required to convert the data storage material layer to the insulator is referred to as a reset voltage, and the voltage required to convert the data storage material layer to the conductor is referred to as a set voltage.
Resistance RAM devices are disclosed in U.S. patent Publication Nos. US 2003/00011789 A1, US 2003/0148545 A1 and US 2003/0003674 A1 as well as U.S. Pat. No. 6,583,003 B1. According to these conventional devices, a praseodymium calcium manganese oxide (Pr,Ca)MnO3) layer (hereinafter, referred to as a “PCMO” layer) is used as the data storage material layer. However, it is difficult to pattern the PCMO layer using a conventional photolithography/etch process, which is widely used in the fabrication of semiconductor devices. This patterning complexity may lead to difficulties in improvement of integration density of resistance RAM devices employing a PCMO layer. In addition, four kinds of materials should be mixed to form the PCMO layer. Thus, it may be difficult to form the PCMO layer having a uniform composition throughout the semiconductor substrate. Moreover, a ZnO-based data storage material layer is disclosed in U.S. Pat. No. 4,472,296 to Hunter, Jr. et al. According to Hunter, Jr. et al., the ZnO-based data storage material layer has a switching characteristic at a high voltage of about 50 V.